The systemic immune-inflammation index (SII) has been implicated in various pathological conditions; however, its role in postoperative sleep disturbance (PSD) remains unclear. This prospective cohort study aimed to investigate the association between SII and PSD in elderly patients undergoing total joint arthroplasty. A total of 500 patients (median age: 70 years, IQR: 67-75) were enrolled between August 2023 and October 2024. The optimal SII cut-off value was determined as 500.3 using receiver operating characteristic curve analysis. Patients with SII >= 500.3 exhibited a significantly increased risk of PSD, with an odds ratio (OR) of 3.26 (95%CI: 2.23-4.75) in univariate analysis. The association remained robust in three multivariate regression models, with adjusted ORs of 2.14 (95%CI: 1.39-3.30), 4.05 (95%CI: 2.68-6.10), and 2.72 (95%CI: 1.71-4.33). Subgroup analyses further confirmed a consistent relationship between elevated SII and PSD across different anesthesia timings, frailty status, preoperative sleep disturbances, and perioperative blood transfusion. Given that SII is derived from routine blood tests, it represents a cost-effective and accessible biomarker for PSD risk stratification. These findings suggest that SII may serve as a valuable predictive marker for PSD in elderly patients undergoing total joint arthroplasty, warranting further validation in larger prospective studies.